Is It Time for an Oil Change in the Kitchen?

WHAT’S the best oil for everyday frying? Some markets where I shop offer more than a dozen oils, from argan and avocado to tea seed and walnut. I’d long figured that the choice is a matter of taste and price. I usually use canola oil because it’s neutral in flavor, a good source of omega-3s and inexpensive. Like soy oil, it costs about a dime a tablespoon, whereas extra-virgin olive oils can run well over a dollar.

Partisans of the olive maintain that a high-quality extra-virgin oil brings its special flavor and health benefits to foods cooked in it. More recipes now suggest it for frying and other high-heat techniques, not just for last-minute drizzling. But does it make enough difference that it’s worth a tenfold premium in price?

I investigated the flavor question by heating 15 oils — 4 olive and 11 seed oils — with nothing else in the pan, so I could taste what heat alone does to them. And I served some of them to trained oil judges.

We were surprised at how thoroughly heat obliterated the flavors in cooking oil until they all tasted more or less the same. Even prize-winning, and costly, extra-virgin olive oils lost much of what makes them special, though they retain their apparently healthful pungency. To get food with the green and fruity flavor of good olive oil, it seems more economical and effective to fry with an inexpensive refined oil and drizzle on a little fresh olive oil after cooking.

Many oils have little or no flavor to begin with, as they’ve been refined to remove almost everything except the oil molecules. This is true of most oils extracted from seeds, including canola and soy. Fresh out of the bottle, the nine refined seed oils I tested were almost odorless. Some seed oils, including peanut and sesame, are also sold in unrefined or partly refined form. These are usually darker and can carry the flavor of their sources. They’re also more sensitive to heat than refined oils. They start breaking down, developing unpleasant flavors and giving off smoke at lower temperatures. Heated in a frying pan, the two unrefined seed oils I tested began to smoke between 375 and 390 degrees, at the upper end of the frying range. The refined oils didn’t start smoking until 475 degrees or higher.

When heated to a moderate frying temperature of 350 degrees, only the unrefined sesame oil had a distinctive flavor. The other 10 seed oils tasted about the same, slightly nutty and, well, fried.

Unlike seed oils, olive oils are pressed from fresh fruits, so their flavors can vary tremendously. Of the four tested, one was an inexpensive “light” olive oil, made primarily of neutral refined oil, with very little aroma.

The other three were labeled “extra virgin,” a standard that in theory signifies an unrefined oil of good quality but in practice doesn’t signify much at all. The first two were a fruity Spanish oil and a spicy, pungent one from California. Both were international medal winners and priced accordingly, at a dollar or more a tablespoon. The third was a suspiciously inexpensive bottle from an upscale supermarket, a blend from several Mediterranean countries. It smelled stale and had a strong odor of fermented olives. These qualities should have disqualified it from extra virgin status because they indicate that the oil was made from damaged fruit.

But oil appeal is on the palate of the taster. According to a forthcoming study from researchers at the University of California, Sonoma, many California consumers actually like and expect these off flavors in olive oils, probably because they’re used to them and have had little or no experience of fresh, well-made oils.

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Credit
Jason Lee

The refined olive oil and two of three extra-virgin olive oils I tested began to smoke at a respectable 450 degrees. The inexpensive extra-virgin oil started to smell of rubber and plastic almost as soon as it became warm, and fumed at 350 degrees.

After I’d heated them, none of the olive oils had much olive flavor left. In fact, they didn’t taste much different from the seed oils.

To get a set of more expert second opinions, I took the olive oils to a meeting of the University of California’s olive oil research group. This panel of trained tasters evaluates oils from all over the world to provide guidance to California’s young olive-oil industry.

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In a blind tasting of the four unheated olive oils, the six tasters easily distinguished the medal winners from the cheaper oils and found many interesting aroma notes in them, from tea and mint to green banana, stone fruit and cinnamon.

For the second blind tasting, I heated each oil to 350 degrees for five minutes. I also heated a sample of the Spanish oil more gently, to 300 degrees, to see whether it might retain more olive flavor.

The panelists said nothing as they swirled and sniffed the heated oils in their small tasting glasses, tinted blue to eliminate any consideration of color, then sipped, slurped and spat. The first spoken comment, immediately seconded by most of the panel members, was, “These oils all taste like popcorn.” In fact the panel ranked the heated light oil higher than the heated pricey California extra-virgin oil, whose pungency was no longer balanced by a spicy aroma and had become overbearing.

Even the defective supermarket oil had become much less offensive. This surprise led one panelist to recall that heating is part of the refining process that manufacturers use to deodorize raw oils. Cooking clearly also drives aromas out of the oil and into the air. That helps explain the harsh smell that filled our kitchen decades ago whenever my mother started to make spaghetti sauce. I hated that aroma, which came from the poor oil she must have used, but I loved her spaghetti sauce.

While it’s understandable that many people have learned to enjoy off flavors in oils, there’s a good reason to recognize staleness and rancidity for what they are and avoid them.

All cooking oils are fragile. Fresh oil begins to deteriorate as soon as it’s exposed to light, heat, oxygen or moisture, all of which can break intact oil molecules into fragments. One set of fragments is responsible for the hints of cardboard, paint and fish that we smell in stale, rancid oil.

It turns out that stale aromas, pleasant fried aromas and unpleasant scorched aromas all come from oil fragments called aldehydes that are more or less toxic to our cells, whether we eat them or inhale them during cooking. Frequent exposure to frying fumes has been found to damage the airways of both restaurant and home cooks. Fresh oils, and in particular fresh olive oils, generate the fewest toxic aldehydes.

So the choice of everyday frying oil should indeed be a matter of taste. Choose a cheap or expensive oil as you like. Fans of extra-virgin olive oil willingly pay more for its provenance and polyphenols as much as its aroma. But learn to taste the difference between good fresh oils and stale or funky ones. Buy small containers that you’ll use up in a few weeks, keep them dark and cool, and taste before you fry.

Correction: November 24, 2010

Because of an editing error, an article last Wednesday about the flavor of cooking oils misidentified the city in California where a meeting of the University of California’s olive oil research group was held. It was in Sonoma, not Davis.

A version of this article appears in print on November 17, 2010, on Page D1 of the New York edition with the headline: Is It Time for an Oil Change?. Order Reprints|Today's Paper|Subscribe